A typical diesel engine injects and supplies fuel around a compression top dead center. On the other hand, a homogeneous charge compression ignition (HCCI) engine is of a type that fuel is injected into a combustion chamber at an early stage or fuel is mixed with air in an intake port, so that premixed gas is self-ignited around a compression top dead center by a combustion reaction at a compression temperature.
Such an HCCI combustion system that dilute premixed gas is burned by self-ignition almost simultaneously in an entire combustion chamber has the following features. That is, the HCCI combustion system achieves high combustion efficiency which is comparable to a diesel engine, generates few soot like a gasoline engine, and considerably suppresses generation of NOx.
In order to realize the HCCI combustion, there have been widely studied an engine based on a gasoline engine and an engine based on a diesel engine.
The former engine uses gasoline as fuel. In a high-load operation and a low-load operation, the engine performs spark ignition combustion like a conventional gasoline engine. In a middle-load HCCI operation, on the other hand, the engine utilizes a large amount of high-temperature EGR gas in order to compensate a low compression ratio and to achieve a compression temperature required for self-ignition. When the high-temperature EGR gas is fed into the combustion chamber, fresh air corresponding to a volume of the fed high-temperature EGR gas is reduced, resulting in considerable reduction of a mass of entire mixed gas added with the EGR gas. Thus, a thermal capacity of the mixed gas decreases while a combustion temperature increases, so that a large amount of NOx is generated. Consequently, a supply amount of fuel can not be increased.
The latter engine uses gas oil as fuel. In a high-load operation and an extremely low-load operation, the engine performs diesel combustion like a conventional diesel engine. In a middle-load operation, on the other hand, the engine advances a fuel injection timing and, then, suppresses ignition by feed of a large amount of cooled EGR gas to ensure a sufficient premixing time until the ignition. Thus, the engine realizes HCCI or combustion close to the HCCI. However, since fresh air is reduced in amount due to the feed of the large amount of EGR gas, soot is generated due to lack of oxygen. Consequently, a supply amount of fuel can not be increased. Herein, the ignition can also be suppressed by lowering a compression ratio. However, such a compression ratio can not be lowered so much because startability and low-temperature combustion become deteriorated.
For these reasons, if such an HCCI engine is used for an automobile, an HCCI operation is limited to a low-to-middle load region due to a main factor that the amount of fresh air is reduced by feed of the EGR gas. Consequently, there arises a problem that improvement in fuel economy and performance of exhaust gas in HCCI can not be achieved satisfactorily in actual run.
JP 51-034526 B, JP 2005-061325 A and JSAE20045094 propose a method of opening an exhaust valve again in an intake stroke to increase an amount of EGR gas at a high load in which feed of the EGR gas becomes difficult. On the other hand, Non-patent document 1 proposes a method of feeding high-temperature EGR gas from an exhaust port into a cylinder at a low temperature.